Okay, this daily build (netbsd-3-0/200605220000Z) is displaying the  
symptoms.
It doesn't appear to be in rpcbind or ypbind itself, but an  
associated library.
It seems when the query is made the the ypserv process on the server  
will log each
user id in the password file when the -l flag is on.  Interestingly  
the userids are
sorted in alphabetical order .. any reason for this? ;-)